The present invention generally relates to liquid crystal display devices, and more particularly to a liquid crystal display device having a MOS (metal-oxide-semiconductor) capacitor and a fabrication process thereof. Further, the present invention relates to such a MOS capacitor, a semiconductor device having such a MOS capacitor, and a fabrication process of those.
Conventionally, liquid crystal display devices have been used widely in portable information processing apparatuses such as so-called notebook computer, as a low-power-consuming compact information display device.
On the other hand, application of a liquid crystal display device is by no means limited to such a portable information processing apparatus. Today, liquid crystal display devices are used also in desk-top type information processing apparatuses as replacement of conventional CRT display device. Further, liquid crystal display devices are attractive as a display device of high-definition televisions (HDTV). Particularly, application to a projection-type HDTV display device is studied.
In the case of such high-performance, large-area liquid crystal display devices, a simple matrix driving construction used conventionally cannot provide satisfy the required specification in terms of response speed, contrast ratio, color purity, and the like. Thus, in such high-performance, large-area liquid crystal display devices, an active-matrix driving method is used in which each pixel is driven by a corresponding thin-film transistor (TFT). In the liquid crystal display device that employs an active-matrix driving method, it has been practiced to use an amorphous silicon liquid crystal display device in which amorphous silicon is used for the active region of the TFT. On the other hand, amorphous silicon has a drawback of small electron mobility and cannot satisfy the specification required for such a high-performance liquid crystal display device. Thus, there is a need of using a polysilicon TFT as the TFT of a high-performance liquid crystal display device.
Generally, a liquid crystal display device that uses the active matrix driving uses a capacitor for each TFT for retaining a driving voltage applied to a liquid crystal layer. Such a capacitor may be formed by a dielectric film sandwiched by a pair of metal electrodes similarly to ordinary capacitors. On the other hand, in view of the fact that the capacitor is used in cooperation with a highly miniaturized TFT, it is advantageous to construct the capacitor to have a so-called MOS structure.
FIG. 1 shows the general construction of a conventional active-matrix driven liquid crystal display device.
Referring to FIG. 1, the liquid crystal display device includes a TFT glass substrate 1A carrying thereon a number of TFTs and transparent pixel electrodes cooperating thereto and an opposing glass substrate 1B formed on the TFT substrate 1A. Between the substrate 1A and the substrate 1B, a liquid crystal layer 1 is confined by means of a seal member 1C. In the illustrated liquid crystal display device, the transparent pixel electrodes are selectively driven via a corresponding TFT and the orientation of liquid crystal molecules is changed selectively in the liquid crystal layer in correspondence to the selected pixel electrode. Further, polarizers not illustrated are disposed at respective outer sides of the glass substrates 1A and 1B. Further, a molecular alignment film not illustrated is formed on the inner sides of the glass substrates 1A and 1B in contact with the liquid crystal layer 1. The molecular alignment film thereby restricts the orientation of the liquid crystal molecules.
FIG. 2 shows a part of the TFT glass substrate 1A in an enlarged scale.
Referring to FIG. 2, the glass substrate 1A carries thereon a number of pad electrodes 13A, to which a scanning signal is supplied, and a number of scanning electrodes 13 extend therefrom, wherein the glass substrate 1A further carries thereon a number of pad electrodes 12A, to which a video signal is supplied, and a number of signal electrodes 12 extend therefrom. The scanning electrodes 13 and the signal electrodes 12 extend in such a manner that an elongating direction of a scanning electrode 13 intersects generally perpendicularly to an elongating direction of a signal electrode 12. Further, TFTs 11 are formed at the intersections of the scanning electrodes 13 and the signal electrodes 12. Further, the substrate 1A carries transparent pixel electrodes 14 thereon such that a pixel electrode 14 corresponds to each of the TFTs 11, and each TFT 11 is selected by a scanning signal on a corresponding scanning electrode 13. Thereby, the selected TFT 11 drives the cooperating transparent pixel electrode 14 by a video signal on the corresponding signal electrode 12.
FIG. 3 shows the construction of a liquid crystal cell driving circuit for driving one pixel of the liquid crystal display device of FIG. 2.
Referring to FIG. 3, a number of liquid crystal cells 15 are formed in the liquid crystal layer 1 of FIG. 1 in correspondence to the plurality of pixels, and it can be seen that a number of the TFTs 11 are formed on the TFT substrate, which corresponds with the glass substrate 1A of FIG. 1, in a row and column formation in correspondence to the liquid crystal cells 15. Further, it can be seen that the signal lines 12 supplying the video signals to the TFTs 11 extend on the TFT substrate 1A in a column direction in a substantially parallel relationship with each other. Further, it can be seen that the gate electrodes (scanning electrodes) 13 controlling the TFTs 11 extend substantially parallel with each other. In the illustrated example, a TFT 11 is formed of a pair of serially connected TFTs 11A and 11B and drives the corresponding liquid crystal cell 15 via the pixel electrode 14. Further, a capacitor 16 is connected to the TFT 11 parallel to the liquid crystal cell 15. The capacitor 16 thereby constitutes an accumulating capacitance holding the driving voltage applied to the liquid crystal cell 15. In the construction, the capacitor 16 is connected between the pixel electrode 14 and a capacitance line 17.
As explained before, the accumulating capacitance 16 may be constructed by sandwiching a dielectric film between a pair of metal electrode patterns. In the case of an active-matrix driven liquid crystal display device, however, it is more advantageous to construct the same in the form of a MOS capacitor.
FIG. 4 shows the circuit construction of a conventional liquid display device that has such a MOS capacitor.
Referring to FIG. 4, the liquid crystal cell is formed of a glass substrate 10A corresponding to the foregoing TFT substrate 1A, a polysilicon pattern 10B formed on the glass substrate 10A, and an oxide film 10C formed on the glass substrate 10A so as to cover the polysilicon pattern 10B. The TFT 11 is formed of n+-type diffusion regions 10a, 10b and 10c formed in the foregoing polysilicon pattern 10B, a gate electrode 11a of Al or polysilicon formed on the oxide film 10C between the foregoing diffusion regions 10a and 10b, and a gate electrode 11b formed similarly of Al or polysilicon on the oxide film 10C between the diffusion regions 10b and 10c. It should be noted that the gate electrode 11a corresponds to the foregoing TFT 11A and the gate electrode 11b corresponds to the foregoing TFT 11B. Further, the oxide film 10C constitutes a gate insulation film underneath the gate electrodes 11a and 11b. Further, the signal line 12 is connected to the diffusion region 10a and the gate control line 13 is connected to the gate electrodes 11a and 11b. 
In the construction of FIG. 4, it can be seen that the diffusion region 11c extends in the right direction in the drawing and forms an n+-type diffusion region 10d. Further, an electrode 11c of Al or polysilicon is formed on the oxide film 10C in correspondence to the diffusion region 10d similarly to the gate electrodes 11a and 11b, wherein the electrode 11c thus formed constitutes a capacitor electrode of the foregoing capacitor 16.
In the liquid crystal display device of such a construction, the TFTs 11A and 11B are turned on in response to the selection signal on the gate bus line 13 and the capacitor 16 is charged by the video signal on the signal line 12 via the diffusion region 10d. As a result, the potential of the pixel electrode 14 connected to the diffusion region 10c and the diffusion region 10d is maintained at a predetermined driving level until the next selection signal comes in.
On the other hand, such a conventional liquid crystal display device has a drawback in the point that, while it allows a self-aligned formation of the diffusion regions 10a, 10b and 10c by using the gate electrodes 11a and 11b as a mask, does not allow a self-aligned formation for the diffusion region 10d. Thus, in order to form the diffusion region 10d, it is necessary to conduct an ion implantation process separately to the diffusion regions 10a-10c by using a separate mask.
However, the use of such a separate mask process and separate ion implantation process for forming the diffusion region 10d increases the number of fabrication steps significantly. Further, there is a risk of mask alignment error, which may lead to the problems such as variation of the threshold value or increase of number of the defective devices. In the construction of FIG. 2, it is possible to abandon the use of self-alignment process for forming the diffusion regions 10a-10c and use the same mask process used for forming the diffusion region 10d. However, such a process has a drawback, due to the fact that the formation of the oxide film 10C is conducted after the ion implantation process, that the surface of the polysilicon pattern 10B is tend to be contaminated by impurity elements. In the case of fabricating a semiconductor integrated circuit, such impurity elements can be eliminated by conducting a cleaning process. In the case of a liquid crystal display device that uses a glass substrate, on the other hand, thorough cleaning process cannot be used and the impurity elements tend to remain on the polysilicon pattern once the impurity element has caused a contamination.
FIG. 5 shows the construction of another conventional liquid crystal display device that eliminates the problem of the liquid crystal display device of FIG. 4. In FIG. 5, those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 5, the illustrated liquid crystal display device includes an n+-type diffusion region 10e in the polysilicon pattern 10B in addition to the n+-type diffusion regions 10a-10c constituting the TFTs 11A and 11B, by a self-alignment process that uses the gate electrodes 11a, 11b and the capacitor electrode 11c as a mask. Thus, the problems of increased number of fabrication steps and the contamination of the polysilicon pattern 10B by impurity element are avoided. In the construction of FIG. 3, a predetermined voltage is applied to the electrode 11c via the capacitance line 17 and a surface accumulation layer is inducted in the polysilicon pattern 10B in the intrinsic or low-doped region 10f between the diffusion regions 10c and 10e. It should be noted that the region 10f has an impurity concentration similar to the channel region formed in the polysilicon pattern 10B between the diffusion regions 10a and 10b or between the diffusion regions 10c and 10d. 
Thus, the construction of FIG. 5 can avoid the problem pertinent to the construction of FIG. 4. On the other hand, the construction of FIG. 5 requires another power source for driving the capacitance line 17 and for inducing a surface accumulation layer in the region 10f, and the construction of the driving circuit of the liquid crystal becomes complex. Thereby, the problem of increase of the manufacturing cost cannot be avoided. Further, as can be seen from the circuit diagram of FIG. 3, the capacitance line 17 thus applied with a high voltage crosses the signal line on the TFT substrate 10F. As there exists only a thin interlayer insulation film between the capacitance line 17 and the signal line 12, there is a risk that a leakage current or breakdown of insulation occurs. It should be noted that the voltage applied to the capacitance line 17 is a voltage much higher than the voltage used commonly in an ordinary semiconductor integrated circuit. Further, in view of the fact that the high voltage is applied continuously to the capacitance line 17, the gate oxide film 10C tends to degrade more than a gate oxide film used in an ordinary MOS transistor. Thus, the capacitor 16 of FIG. 5 has a drawback in terms of reliability.
Further, the construction of FIG. 5 has a drawback, associated with the application of high voltage to the capacitance line 17, in that there tend to be formed domains in the liquid crystal cell in correspondence to the capacitance line and other wiring or TFT. In order to avoid disturbance of representation associated with such a domain formation, it is necessary to provide an opaque mask along the capacitance line with a substantial width. However, formation of such a wide opaque mask causes a decrease of aperture ratio of the liquid crystal display device.
Accordingly, it is a general object of the present invention to provide a novel and useful MOS-type capacitance device, a liquid crystal display device, a semiconductor device, and a fabrication process thereof, wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a MOS-type capacitance device having a simple construction for easiness of fabrication, a liquid crystal display device having such a MOS-type capacitance device, and a fabrication process thereof.
Another object of the present invention is to provide a MOS-type capacitance device, characterized by:
a substrate,
a semiconductor layer formed on the substrate,
an insulating film formed on the semiconductor layer,
an electrode formed on the insulating film,
a first diffusion region formed in the semiconductor layer adjacent to the electrode, and
a second diffusion region formed in the semiconductor layer adjacent to the electrode,
the first diffusion region being doped to a first conductivity type, the second diffusion region being doped to a second, opposite conductivity type.
Another object of the present invention is to provide a MOS-type capacitance device, characterized by:
a substrate,
an electrode formed on the substrate,
an insulating film formed on the substrate so as to cover the electrode,
a semiconductor layer formed on the insulating film,
a first diffusion region formed in the semiconductor layer adjacent to a first edge of the electrode,
a second diffusion region formed in the semiconductor layer adjacent to another edge of the electrode,
the first diffusion region being doped to a first conductivity type, the second diffusion region being doped to a second, opposite conductivity type.
According to the present invention, the MOS-type capacitance device shows substantially the same capacitance with regard to a positive voltage or a negative voltage, or with regard to a low-frequency signal or a high-frequency signal, by forming a complementary connection. The MOS-type capacitance device thereby performs as an effective capacitor. Further, the MOS-type capacitance device of the present invention can be formed simultaneously with the fabrication process of other MOS transistors, without adding fabrication steps. In the MOS-type capacitance device of the present embodiment, the type diffusion region and the p+-type diffusion region are formed by conducting an ion implantation process after covering the semiconductor layer by the insulating film. Thereby, the problem of contamination of the semiconductor layer by impurity elements occurring in the conventional art is eliminated. Associated with this, the problem of variation of the threshold voltage or other operational characteristics, caused in the transistors that are formed on the semiconductor layer simultaneously to the MOS-type capacitance device, as a result of the contamination of the impurity elements, is eliminated. When the MOS-type capacitance device of the present invention is used for driving a liquid crystal display device, the capacitor electrode may be simply held at a common potential level. Thereby, the stress applied to the capacitor insulation film or other interlayer insulation film is reduced and the degradation of display characteristics arising from the stress is avoided.
Another object of the present invention is to provide a liquid crystal display device, characterized by:
a first glass substrate, a second glass substrate opposing the first glass substrate,
a liquid crystal layer confined between the first glass substrate and the second glass substrate,
a signal electrode extending on the first glass substrate,
a scanning electrode extending on the first glass substrate,
a common potential line extending on the first glass substrate,
a thin-film transistor formed at an intersection of the signal electrode line and the scanning electrode,
a pixel electrode electrically connected to the thin-film transistor,
and an accumulating capacitance connected parallel to the pixel electrode,
the thin-film transistor being formed in a semiconductor layer formed on the first glass substrate,
the accumulating capacitance comprising:
an insulating film formed on the semiconductor layer,
a capacitor electrode formed on the insulating film,
a first diffusion region formed in the semiconductor layer adjacent to the capacitor electrode, and
a second diffusion region formed in the semiconductor layer adjacent to the capacitor electrode,
the first diffusion region being doped to a first conductivity type, the second diffusion region being doped to a second, opposite conductivity type.
According to the present invention, it is possible to reduce the manufacturing cost of the liquid crystal display device by using the MOS-type capacitance device in the liquid crystal display device. Further, the liquid crystal display device has an advantageous feature of improved reliability and high yield of production due to reduction of stress applied to a gate insulation film, the capacitor insulation film or other interlayer insulation films.
Another object of the present invention is to provide a projection-type liquid crystal display device, characterized by:
an optical source,
a liquid crystal panel disposed in an optical path of an optical beam produced by the optical source for spatial modulation, and
a projecting optical system projecting the optical beam that has been spatially modulated by the liquid crystal panel,
the liquid crystal panel comprising:
a first glass substrate,
a second glass substrate opposing the first glass substrate,
a liquid crystal layer confined between the first glass substrate and the second glass substrate, a signal electrode extending on the first glass substrate,
a scanning electrode extending on the second glass substrate,
a common potential line extending on the first glass substrate,
a thin-film transistor formed at an intersection of the signal electrode line and the scanning electrode,
a pixel electrode electrically connected to the thin-film transistor, and
an accumulating capacitance connected parallel to the pixel electrode,
the thin-film transistor being formed in a semiconductor layer formed on the first glass substrate,
the accumulating capacitance comprising:
an insulating film formed on the semiconductor layer,
a capacitor electrode formed on the insulating film,
a first diffusion region formed in the semiconductor layer adjacent to the capacitor electrode, and
a second diffusion region formed in the semiconductor layer adjacent to the capacitor electrode,
the first diffusion region being doped to a first conductivity type, the second diffusion region being doped to a second, opposite conductivity type.
According to the present invention, it becomes possible to manufacture a projection-type liquid crystal display device using a MOS-type capacitance device at low cost without increasing the number of manufacturing steps. The liquid crystal display device has an advantageous feature of improved reliability and high yield of production due to reduction of stress applied to a gate insulation film, the capacitor insulation film or other interlayer insulation films.
Another object of the present invention is to provide a semiconductor integrated circuit device having a capacitor, the capacitor comprising a substrate, an insulating film formed on the substrate, an electrode formed on the insulating film, a first diffusion region formed in the substrate adjacent to the electrode, and a second diffusion region formed in the substrate adjacent to the electrode, characterized in that the first diffusion region is doped to a first conductivity type and the second diffusion region is doped to a second, opposite conductivity type.
According to the present invention, various semiconductor integrated circuits can be manufactured by using the MOS-type capacitance device.
Another object of the present invention is to provide a method of fabricating a MOS-type capacitance device, characterized by the steps of:
forming a semiconductor film on a substrate,
forming an insulating film on the semiconductor film,
forming a gate electrode on the insulating film,
introducing an impurity element of a first conductivity type into the semiconductor film at a first side of the gate electrode while using the gate electrode as a mask, and
introducing an impurity element of a second, opposite conductivity type into the semiconductor film at another side of the gate electrode while using the gate electrode as a mask.
Another object of the present invention is to provide a method of fabricating a liquid crystal display device having a MOS-type capacitance, characterized in that the MOS-type capacitance is fabricated according to the steps of:
forming a semiconductor film on a glass substrate,
forming an insulating film on the semiconductor film,
forming a gate electrode on the semiconductor film,
introducing an impurity element of a first conductivity type into the semiconductor film at a first side of the gate electrode while using the gate electrode as a mask, and
introducing an impurity element of a second, opposite conductivity type into the semiconductor film at another side of the gate electrode while using the gate electrode as a mask.
Another object of the present invention is to provide a method of fabricating a liquid crystal display device having a MOS-type capacitance device, characterized in that the MOS-type capacitance device is fabricated according to the steps of:
forming a capacitor electrode on a glass substrate,
forming an insulating film on the glass substrate so as to cover the capacitor electrode,
forming a semiconductor film on the insulating film,
introducing an impurity element of a first conductivity type into the semiconductor film at a first side of the gate electrode, and
introducing an impurity element of a second, opposite conductivity type into the semiconductor film at another side of the gate electrode.
Another object of the present invention is to provide a method of fabricating a semiconductor integrated circuit device having a MOS-type capacitance device, characterized in that the MOS-type capacitance device is fabricated according to the steps of:
forming an insulating film on the semiconductor substrate,
forming a gate electrode on the insulating film,
introducing an impurity element of a first conductivity type into the semiconductor substrate at a first side of the gate electrode while using the gate electrode as a mask, and
introducing an impurity element of a second conductivity type into the semiconductor substrate at another side of the gate electrode while using the gate electrode as a mask.
According to the present invention, it becomes possible to fabricate a MOS-type capacitance device, or a liquid crystal display device using the same, or a semiconductor integrated circuit device using the same.